pistons - the shape of things to come

Status
Not open for further replies.
Ken, so why is the "norton rod" small end so light? you would think that high performance parts makers would exceed stock parts for BOTH strength and weight.

My only guess at the answer would be that the norton rod gets it's light weight at the cost of strength and that the other rods are significantly stronger. Sorry to be a pain... Maybe you or Jim will explain something I'm missing...
 
Speedway JAWA engines are probably the most extreme case of a long stroke racing motorcycle engine. I think they have aluminium rods. On the dirt, they buzz along at about 8000 RPM.
With the Commando engine, I have only ever worried about leaving a piston behind - but that does not seem to be the major problem with high revs - it is the crank.
 
o0norton0o said:
Ken, so why is the "norton rod" small end so light? you would think that high performance parts makers would exceed stock parts for BOTH strength and weight.

My only guess at the answer would be that the norton rod gets it's light weight at the cost of strength and that the other rods are significantly stronger. Sorry to be a pain... Maybe you or Jim will explain something I'm missing...
Click to expand...
The Norton rod is good because its forged but its too short for the stroke and that's part of the vibration problem. There is no other forged aluminum rod available for Nortons and todays high performance motors and big bores have exceeded the strength limitations of the stock rod. Part of the reason its light is because its short and the heavy steel cap acts like a counterweight on the balancer which makes the small end appear lighter than it actually is. If you weigh the small end of a Norton rod without the steel cap and compare it to the small end weight of a JS bushless steel Carrillo rod without a cap I think you will find them to be much closer. Even so the Carrillo is much stronger and more resistant to fatigue.

The cap weight doesn't effect the reciprocating weight balance (only the rotating weight). If you change the cap weight and weigh the small end on a balancer it will show different results - even though you haven't actually changed the mass of the small end. This is a deceptive problem in calculating actual small end rod weights. In this case our limited science is lying to us.

For instance I could design a heavy Carrillo rod cap and make the small end appear to be lighter than a stock Norton small end.

Years ago Ken mentioned an aluminum matrix rod that had lightweight promise but price and availability were a problem.
 
thanks for the explanation Jim. As usual there are no single variable equations in physics, so there's no understanding without seeing the bigger picture beyond a single component's quality.
 
+1 for Jim's explanation. It's as good as you can get without some much more complicated physics. Weighing the rod ends to decide how much to call reciprocating and how much rotating is an approximation that works quite well in balancing calculations, even though it's not technically correct. And yes, I also am quite sure that the stock rod gets it's lightness at the expense of strength, and that the heavier steel rods are much stronger. That doesn't mean it isn't suitable for street Commando use, unless one is making a lot more power, running at redline a lot, or doing huge mileage. In those cases, you will probably eventually exceed the strength and/or fatigue limits of the rod. One of the major advantages of the steel rod is that it will never reach it's fatigue limit under any sort of realistic conditions, where the aluminum rods will.

Ken
 
Why have I seen steel con-rods from two-strokes which have failed ? My friend broke one in a TZ750 as he crossed the finish line at Phillip Island at extremely high speed. - Anything will break if you over-rev it, or you run it slack. I suggest a steel con-rod does not allow you to increase your rev limit. And that is how some guys try to get more horsepower, instead of fattening-up the mid range. The balance factor sets the revs at which the engine will be smooth. An Atlas engine is smooth at 8000 RPM, when the balance factor is about 80 %.
 
Last edited:
jseng1 said:
Ken
Another rod you can add to your list is the Ultralight JS rod with only 95 grams on the small end. Its stronger than a stock Norton rod and is rated for 75 hp by Carrillo with their safety margin for endurance racing type applications. Recommended for solid frame 750s etc.
Click to expand...

Hi Jim,
Why are your ultralight rods recommended for rigidly mounted engines only? I can't think that the additional dynamic peak loading of approx. 600 N @ 8000 rpm at the small conrod end of a rubber mounted engine plays a significant role given that the combustion peak force at the conrod end is about 25000 N. The maximum deceleration force at TDC is about 16500 N (tensile load on rod) . The difference between these two loads is 8500 N (compressive load) roughly speaking. The additional dynamic load makes up 7% and there is a phase change on top of that. True, the dynamic loads acts at every rotation while the combustion load acts every second rotation only. There are also dynamic compressive loads at BDC which we need to to account for in a fatigue calculation (peak value about 8000 N compressive load). It's hard to believe Carillo optimized the rods to such a degree that the ISO mounting will impair on fatigue life of the rods. Please explain.

What is the life expectancy in a rigidly mounted engine as calculated by Carillo?

-Knut
 
When you fit a race cam into a near standard motor you usually get a more pronounced power band. You also get more torque both below and right through the usable rev range. The reciprocating weight sets the limit on how high you can safely rev the motor. If you have not made the internals stronger and lighter, you cannot get more power by revving the motor higher.
 
mdt-son said:
Hi Jim,
Why are your ultralight rods recommended for rigidly mounted engines only? I can't think that the additional dynamic peak loading of approx. 600 N @ 8000 rpm at the small conrod end of a rubber mounted engine plays a significant role given that the combustion peak force at the conrod end is about 25000 N. The maximum deceleration force at TDC is about 16500 N (tensile load on rod) . The difference between these two loads is 8500 N (compressive load) roughly speaking. The additional dynamic load makes up 7% and there is a phase change on top of that. True, the dynamic loads acts at every rotation while the combustion load acts every second rotation only. There are also dynamic compressive loads at BDC which we need to to account for in a fatigue calculation (peak value about 8000 N compressive load). It's hard to believe Carillo optimized the rods to such a degree that the ISO mounting will impair on fatigue life of the rods. Please explain.

What is the life expectancy in a rigidly mounted engine as calculated by Carillo?

-Knut
Click to expand...

My motor is rigidly mounted and with a crank balance factor of 70% is smooth at 7000 RPM. My friend has an Atlas which has a balance factor of 80% and is smooth at 8000 RPM.
The balance factor in a Commando with isolastics is 58% (?). What are the motor internals doing at 8000 RPM ?
 
A common mistake many guys make when they go racing is to enlarge the inlet ports. Large ports suit high revs. Enlarging them can destroy midrange, which with a Commando engine is extremely important. I use 34mm Amals with Mikuni needles and run methanol fuel, but after 10mm into the cylinder head, the ports are still standard size. I run very high overall gearing, but with a close ratio gear-box.
 
acotrel said:
My motor is rigidly mounted and with a crank balance factor of 70% is smooth at 7000 RPM. My friend has an Atlas which has a balance factor of 80% and is smooth at 8000 RPM.
The balance factor in a Commando with isolastics is 58% (?). What are the motor internals doing at 8000 RPM ?
Click to expand...

The balance factor has no direct bearing on the conrod. As far as motor internals are concerned, that's what I tried to discuss above for the conrod.

-Knut
 
The piston weight affects the balance factor. The conrod is attached to the piston. When the crank is out of balance with the reciprocating weight as happens at different revs to the optimum, you get more stress and vibration - regardless of the isolastics. The isolastics only dampen the vibration externally. A normal Commando would probably have a safer motor, if the crank was balanced for higher revs. But you would have to live with the shake in traffic.
 
acotrel said:
Why have I seen steel con-rods from two-strokes which have failed ? My friend broke one in a TZ750 as he crossed the finish line at Phillip Island at extremely high speed. - Anything will break if you over-rev it, or you run it slack. I suggest a steel con-rod does not allow you to increase your rev limit. And that is how some guys try to get more horsepower, instead of fattening-up the mid range. The balance factor sets the revs at which the engine will be smooth. An Atlas engine is smooth at 8000 RPM, when the balance factor is about 80 %.
Click to expand...

We never said you couldn't break a steel con rod. In fact, I've mentioned breaking one of the factory short stroke 750 steel rods several times on the forum. They were pretty well known for being a bit wimpy.

But I think you'd have to be really abusive to break a Carrillo rod in a Commando, like running a big dose of nitromethane and revving to 9,000 rpm, or something similar.

Ken
 
mdt-son said:
Hi Jim,
Why are your ultralight rods recommended for rigidly mounted engines only? I can't think that the additional dynamic peak loading of approx. 600 N @ 8000 rpm at the small conrod end of a rubber mounted engine plays a significant role given that the combustion peak force at the conrod end is about 25000 N. The maximum deceleration force at TDC is about 16500 N (tensile load on rod) . The difference between these two loads is 8500 N (compressive load) roughly speaking. The additional dynamic load makes up 7% and there is a change on top of that. True, the dynamic loads acts at every rotation while the combustion load acts every second rotation only. There are also dynamic compressive loads at BDC which we need to to account for in a fatigue calculation (peak value about 8000 N compressive load). It's hard to believe Carillo optimized the rods to such a degree that the ISO mounting will impair on fatigue life of the rods. Please explain.

What is the life expectancy in a rigidly mounted engine as calculated by Carillo?

-Knut
Click to expand...

Isolastics are not considered as a factor in fatigue. I'm only concerned about vibration and cutting a few grams to smooth out solid frame 750s and the smaller Domnis.

I've never known of a Carrillo rod to break in a Norton and I've had my standard rods subjected to 90 HP 920 and 1000cc racers and a 150HP 920cc Norton on Nitrous.

The ultralight rod was designed for 750s with less than 75 hp. But it doesn't give enough saftey margin for 80+ hp all out screaming 750 short strokes or pumped up 850s and 920s. What Carrillo said is that it was good for 75 hp endurance racing and that makes it good for long stroke 750 street bikes racers who are concerned about vibration. I don't think it makes sense to put ultralight rods in isolastic frames and I prefer a safety margin.

The ultralight rod is for solid frame Atlas and Domni street bikes and racers (500 600 650 750cc) to reduce vibration as much as possible.
 
jseng1 said:
Isolastics are not considered as a factor in fatigue. I'm only concerned about vibration and cutting a few grams to smooth out solid frame 750s and the smaller Domnis.

I've never known of a Carrillo rod to break in a Norton and I've had my standard rods subjected to 90 HP 920 and 1000cc racers and a 150HP 920cc Norton on Nitrous.

The ultralight rod was designed for 750s with less than 75 hp. But it doesn't give enough saftey margin for 80+ hp all out screaming 750 short strokes or pumped up 850s and 920s. What Carrillo said is that it was good for 75 hp endurance racing and that makes it good for long stroke 750 street bikes racers who are concerned about vibration. I don't think it makes sense to put ultralight rods in isolastic frames and I prefer a safety margin.

The ultralight rod is for solid frame Atlas and Domni street bikes and racers (500 600 650 750cc) to reduce vibration as much as possible.
Click to expand...
I have long been considering your 12 to 1 comp. pistons and long rods. When I get rich again, I will become one of your customers. There must be some extremely fast Commandos in your country. Even without much modification, my bike is fast enough to be very competitive in Australian Period 4 historic races (1963 to 1972). I think the Mk3 Seeley frame must be much lighter than the Commando frame.
 
I'm currently getting around thanks to two of these:

pistons - the shape of things to come


Recommended oil change intervals are nearly 10,000 miles.
 
I think that at 7000 RPM in a normal Commando engine, the piston speed is high enough to get ring flutter. But if you go short stroke, the bike might be more difficult to ride well. What I have noticed with my bike, is it is much easier to ride fast than anything else I have ever been on. - More horsepower is good, but with a race bike it is the whole package which counts. What amazes me is that PW beat the two-strokes with a Commando - the difference in horsepower is enormous. You are talking about 80 BHP max. for the Commando and over 100 BHP for the two-strokes. The rider does not do much other than turn the handle.
 
acotrel said:
A common mistake many guys make when they go racing is to enlarge the inlet ports. Large ports suit high revs. Enlarging them can destroy midrange, which with a Commando engine is extremely important. I use 34mm Amals with Mikuni needles and run methanol fuel, but after 10mm into the cylinder head, the ports are still standard size. I run very high overall gearing, but with a close ratio gear-box.
Click to expand...

Not being a wise guy, just asking. Have you ever ridden a Fred Barlow Spares (FBS) ported Norton with stock valve sizes and a race cam in it? Fred Barlow was pretty good at porting on the inlet side. I don't have what would be considered a race cam. I do have an FBS ported head and a 2S cam in a little 750, and it does not lack anywhere in the power band. I have no idea how that would translate to a 4S or larger cam and really tall gears. My gearing is higher than what my little bike came with, but not that high. I don't use Amal carburetion and use a lot of home garage shade tree engineered parts. My Norton is not what I would consider a fast bike by any stretch of the delusional imagination, and I'm pretty darn delusional. I think my Delta Box FZR 400 was quicker.

Anyhow, a member here named Chris has a Norton prepped for racing with a Fred Barlow ported head, a 4S cam in it, and a bunch of other nice parts. He would know more than I ever will about whether or not a FBS ported Norton head was a mistake on a race bike.

Jim,
I'm glad you are still around doing this stuff. Options are important to the guys that want to go fast.
 
Hi Schwany
Fred had a very good reputation as do most of the classic tuners. I know of nobody who hogs out the intakes or exhausts. However progress & development means new ideas come along. The most important thing with all tuning is that it's a package. Porting without a cam will make a difference with a good cam a difference, but all together! porting cam , carbs, exhaust shape & size. Then you find out how important crank balance is. I have 2 of Steve Maneys heads also a Mez head & a head from an ex Thruxton race shop racer/ engineer. Who raced production. He cleaned the ports up & sunk the inlet valves into the head because he believed the valve masked the flow. Kens Fullato head with Comnoz porting has more downdraft with extra material in the port. I think this is a big plus. All of them are beautifully done! On a road bike match what you do & buy for your engine, to how you want your bike to perform. Don't take advice from someone who runs on methanol.
 
Status
Not open for further replies.
Back
Top